Nobody is quite sure why, but TV shows designed to scare the pants off you – everything from spooky, paranormal and supernatural things to ones that go bump in the night – are now in vogue. This fall, creepy horror will vie with science fiction, urban legends and apocalyptic visions of alien life forms oozing out of pods.

Think of it as must-scream TV.

Six new primetime series are based in paranormal, science fiction and supernatural doings are coming to network near you. Three of the six gurgle with slimy underwater alien life forms.

“That was no fish!” one character shrieks.

No, that was a TV trend.

Social theories abound about television reflecting the zeitgeist, echoing the impact of the terrorist attacks of 9/11 on the American psyche, offering catharsis in troubled times. We may be targets for terrorism, but at least no vampire has attacked lately. As some of these series develop, those explanations may hold water.

The offerings include “Surface,” NBC’s “something is down there” series. It is a mystery-adventure involving a strange aquatic species. The oddity ensnares fishermen, oceanographers and a San Diego family.

“Threshold,” the CBS suspense drama about an alien invasion, starts with an extraterrestrial craft in the Atlantic Ocean.

“Invasion,” on ABC, looks at the inexplicable doings and possible new life forms that appear in a small town in the Florida Everglades after a hurricane.

On land, “Ghost Whisperer,” on CBS, stars Jennifer Love Hewitt as a newlywed who communicates with apparitions who have unfinished business.

“Supernatural,” a road show that’s sort of “X-Files” meets “Route 66,” discovering inexplicable creatures and evil supernatural phenomena in a new town each week, is the most promising hour on the WB’s fall schedule.

And ABC’s “The Night Stalker” is an update of the 1970s Kolchak thrillers, replete with dog-humans, vampires and other violent mysteries. Cue the spectral pipe organ.

Either the writers are all paranoid, or there’s something going on here. Why are there so many paranormal shows?

“It’s a little weird,” concedes Jeff Zucker, president of the NBC Universal Television Group.

For now, practical answers from those in the business are more tenable.

Shaun Cassidy, creator of “Invasion,” says the trend can be explained in “three words: ‘Lost,’ ‘Lost’ and ‘Lost.”‘ The success of that show has encouraged networks to “trust producers to play with unanswered questions,” he said.

The attempt to duplicate the ratings of ABC’s “Lost” spurred a number of these scary, otherworldly shows, known as genre dramas. The relative success of sci-fi thriller “The 4400” on cable and the I-see-dead-people series “Medium” on NBC also may be factors.

Producers and network executives offer explanations for the proliferation of scary-creepy shows that may be surprising to viewers:

The difficulty of cloning “Desperate Housewives.” Because it sprang from the mind of creator Marc Cherry and wasn’t the usual TV-by-committee, “Desperate Housewives” isn’t easy to copy.

The misperception that “Lost” is science fiction and therefore easily imitated. “Really, at its heart is great character work,” ABC boss Stephen McPherson said. “It has very little sci-fi elements, but people thought that’s easy to replicate.”

The declining cost of digital special effects. It’s cheaper to make good scary monsters, outrageous explosions and bewildering otherworldly atmospherics these days.

The cost-effectiveness of horror movies in general is a partial explanation. A tight budget is the mother of invention.

“In that genre, more than any others, you’re more helped by budgetary constraints than you are hurt,” said Eric Kripke, executive producer of “Supernatural.”

“Look at everything from ‘Jaws’ to ‘Night of the Living Dead’ and the more a budget forces you to be evocative – where you actually see less rather than more – the scarier it is,” Kripke said.

He and his team use that to their advantage. “It’s all about, you know, the plinking of water or what’s out there in the darkness.”

What you can’t see can freak you out – at no cost to the producer. In “Supernatural,” where two brothers travel the country investigating demons from urban legend and folklore, including Bloody Mary, the American Indian beast known as the Wendigo and more, viewers may be predisposed to jump off of their couches.

Kripke promises “doppelgangers and succubi and lions and tigers and bears.”

Oh my!

Cassidy (the teen TV and pop star who went on to produce “American Gothic” and “Cold Case”), said the invasion of his new series’ title is “change and how we acclimate against formidable odds.”

“We’re living in an aftermath world,” Cassidy said. The post-9/11 zeitgeist may have been in his subconscious when he was writing the pilot, he said, “but the hard and fast answer is ‘Lost.”‘

Frank Spotnitz (“The X-Files”), writer/executive producer of ABC’s “Night Stalker,” said his thriller is grounded in the same basic tension as the rest of the genre shows: a battle of good versus evil. That leads him to excuse the extreme violence in the pilot by saying, “I believe in showing the effects of violence, and that evil is behind violence.”

Science fiction, horror, paranormal and serial drama are distinct forms. But the various series coming in September play with the formats, sometimes making them difficult to classify. What the best have in common is the unnerving sensation of a thrill ride.

“People love to be scared because it’s a cathartic experience,” Spotnitz said. “By the end of the hour you feel better.”

“Supernatural” is “designed to make it difficult to go to sleep at night after you watch it,” said executive producer McG (“Charlie’s Angels”).

“Your mind is going to be playing tricks on you,” he said, “And that’s just a delicious feeling.”

“Judas,” a new novel by Amos Oz, is a paradox of stillness and provocation. The Israeli author, a long-rumored contender for the Nobel Prize, has reduced the physical action of this story to a tableau of domestic grief.